This invention relates to rare-earth tantalates and niobates and a hydrothermal method for their synthesis. Rare-earth Group 5 oxides (tantalates and niobates) are refractory materials whose syntheses usually require high-temperature, solid-state methods. The rare-earth tantalates and niobates generally have high crystallization temperatures, and the incompatibility between acid-soluble rare-earth oxides and base-soluble tantalum oxides and niobium oxides renders aqueous solution synthetic techniques difficult to implement.
Rare-earth tantalates (RETs) and Rare-earth niobates (RENs) are chemically and electrochemically stable, and RETs and RENs doped with Eu3+ or Tb3+ can be excited by blue or UV light to provide emission in the red or green. However, they had previously been difficult materials to produce without using high-temperature solid-state processing. Well-known drawbacks to solid-state syntheses include difficulties in removing impurities and difficulties in discovering and recovering new phases and/or compositions. Solid-state synthesis is also often not well suited for obtaining controlled nanoparticles or forms suitable for coating applications that would be of great utility in devices such as light-emitting diodes (LEDs).
A simple solution-based synthetic method for rare-earth Group 5 oxides would be highly desirable, but reliable solution methods of synthesizing such materials with good control of the crystalline structure and especially good control of the process for the formation of nanoparticles of the rare-earth Group 5 oxides is lacking.
Wong et al. (U.S. Pat. No. 7,585,474) purports to report a set of methods for making a single crystal ternary nanostructure of composition AxByOz. While Wong claims a sol-based method and a molten-salt method of making an extraordinarily wide range of AxByOz compounds comprising almost every known metal in the periodic table using their methods, they provide no evidence that they were in possession of the crystalline chemical compounds of this present invention or of any rare-earth tantalate or rare-earth niobate. Wong et al. asserts that 69 different elements may be used for each of A or B. However, the materials that they list as having made were restricted to Bi and Fe oxides of formulas BixMyOz and M′xFeyOz where M′ is a transition metal. Materials of formulas BiFeO3 and Bi2Fe4O9 are specifically reported and claimed. Example syntheses reported by Wang et al. are for BiFeO3, Bi2Fe4O9, and BiFeO3—BaTiO3. None of the materials reported to have been made by Wong contain rare-earths and tantalates or rare-earths and niobates.
Given the unpredictable nature of the chemical art, there is no reason to believe that Wong et al. have successfully synthesized any rare-earth tantalates or rare-earth niobates. Therefore, Wong does not report the formation of the materials of this invention, and it does not enable one to fabricate rare-earth tantalates or rare-earth niobates of this present invention. Rather, the teachings of Wong set forth in the specification provided no more than a “plan” or “invitation” for those of skill in the art to experiment to try to obtain single crystalline ternary nanostructures composed of metals other than those actually demonstrated and reported by Wong.